Conditioning ingots



1966 R. J. KRIEGER ETAL 3,231,430

CONDITIONING INGOTS Filed Dec. 28, 1964 INVENTORS Robert J. Krieger Eldon R. Poulsen United States Patent 3,231,430 CONDITIONING INGOTS Robert J. K'rieger, Las Vegas, and Eldon R. Poulsen,

North Las Vegas, Nev., assigors to Titanium Metals Corporation of Alnerica, New York, N.Y., a corporation of Delaware Filed Dec, 28, 1964, Ser. No. 426,462 8 Claims. (Cl. 148-4) This is acontinuation-in-part of our application Serial No 2 131,449, filed July 30, 19 62, now abandoned.

f This invention relates to an improved method for conditioning thesurf'ace of a refractory metal ingot.

The term conditioning is used in the refractory metal art to denote a process in which defects are eliminated from the surface of a body of such metal to place it in proper condition for subsequent processing. The term is so used herein. Ingots of refractory metals as produced, for example, by melting'or powder metallurgy methods often have surface characteristics which are undesirable for subsequent processin Such surfaces may show defects such as cracks, cold shuts, pits, loose metal, condensed impurities, and cruddy deposits. Ingots produced from powder, as well as those produced by melting, should present a clean, smooth, solid surface so that the quality of the products produced from them (for example, forgings) will not be adversely affected. Her'etofore, the conditioning vprocess to improve an ingot surface, particularly the surface of an ingot produced by "arc melting into a cold mold, has generally comprised turning on a lathe to machine off contaminated and undesirable structure metal and also to remove impurities, loose metal and cruddy deposits. This process obviously is expensive, especially when hard and tough metals are being machined. In addition, a substantial amount of the ingot metal must inevitably be removed during such an operation, and the amount removed is rarely below 6 percent of the cast ingot original weight, and may, under adverse conditions, rise to as high as 1'0 percent or 15 percent. True, the machine chips can be processed and remelted, but this involves additional expense; this also increases the danger of contam'ination from raising the impurity level of the product as well as handling and melting difficulties associated with the use of particulate metal, compared to the larger bodies normally' handled. A real need has therefore existed for a simple, economical, and efiicient method for conditioning ingots of refractory metals, and particularly for such a method which would not involve metal removed and loss of cast ingot weight.

It is therefore a principal object of this invention to provide an improved method for conditioning the surface of a refractory metal ingot. Another object of this in.- vention is to provide a method for conditioning the surface of a refractory metal ingot without removing metal from such ingot. Another object of this invention is to provide a method for conditioning the surface of a refractory metal ingot which will provide a smooth, solid surface free of defects which would adversely alfect subsequent processing operations. Another object of this invention is to provide a more economical and efiicient method for conditioning the surface of a refractory metal ingot. These and other objects of this invention will be apparent from the following description thereof.

In its broad aspects, this invention contemplates a method for conditioning the surface of an ingot of refractory metal which comprises heating'a limited area of "the surface of the ingot by impingement thereon of a jet of arc plasma to form a puddle of molten metal on the ingot surface. The torch or gun, as it is sometimes referred. to, used for producing the jet of arc plasma, and the ingot are then caused to move relative to each other so that the puddle of molten metal travels across a surface of the ingot, the advance of the puddle of molten metal involving progressive melting and solidification of increments of the metal of the surface of the ingot. The relative motion between the arc plasma gun and the ingot is continued and arranged so that the ingot surface is eventually covered by the travel path of the puddle of molten metal with individual segments of the puddle travel path continually overlapping. Meanwhile, a flow of inert gas is maintained over the surface of the ingot being conditioned to prevent deleterious oxidation thereof.

The process of this invention may be morereadily understood by reference to the annexed drawings in which:

FIG. 1 illustrates a broken out, side view of apparatus suitably employed for the practice of an embodiment of this invention, particularly for conditioning a cylindrical ingot.

FIG. 2 illustrates a vertical section of the apparatus of FIG. 1 taken along the line 22.

FIG. 3 shows in more detail the angular relationship and positioning of the plasma jet torch with respect to the surface of the ingot being conditioned.

Referring now particularly to FIGS. l and 2, the apparatus comprises a box-like container 10 provided with a valved pipe connection 12 used for initially evacuating container 10 and refilling with inert gas. Valved bleed line 14 is also provided in a wall or top, as shown, of container 10 .to vent excess inert gas introduced during the conditioning operation. A door 16, suitably sealed around its edges when closed, is provided in one end of container 10 for charging and discharging ingots in and out.

Rotatably mounted near the bottom and inside container 10 are horizontal rollers 18. Attached to the ends of the roller shafts are spur gears 20 whose teeth mesh with a common drive pinion 22. Drive shaft 24 on which pinion 22 is attached projects through the end wall of container 10, its passage therethrough being suitably sealed, and connects with reduction gear 26, which in turn is actuate-d by electric motor 28.

An ingot 30, to be conditioned, may be supported as shown on rollers 18 and may be slowly rotated when these are rotated by the train of gears 20 and 22, drive shaft 24, reduction gear 26 and motor .28.

Contact shoe 32 is arranged to bear against one of rollers 18 to conduct current through roller 18 to ingot 39, led into container 10 by insulated flexible conducting cable 34.

Near the top of the inside of container 10 is provided screw shaft 36 whose external end is connected to reduction gear 38 which in turn is connected to motor 40. Threaded onto screw shaft 36 is torch mount 42 prevented from rotating by guide bars 44, and which will-be actuated to-travel back and forth along screw shaft 36 during rotation thereof by motor 40.

Attached to-the'underside of mount 42 is a plasma jet torch 46. Torches of the type required are available commercially, and as is known in the art, operate by maintaining a flow ofgas, generally inert, through an are which may be maintained between a pair of electrodes which are incorporated in the torch, or the arc may principally be maintained between an electrode within the torch and a workpiece, in this instance, ingot 30. Suitable these may be used singly or in admixture.

torch design will provide an eflluent of hot gas are plasma indicated at 48 in FIG. 3. A heat shield or baffle 49 is provided, arranged with a suitable slit for torch travel, to protect the torch mounting, actuating and power and gas lead equipment from the heat of the arc plasma, the power leads 50 and 52. being flexible, insulated. conducting cables connected respectively to the internal or back electrode and the nozzle electrode of torch 46. Inert gas supply to torch 46 is provided by flexible tube 54. Cooling water circulation to the plasma jet torch is provided through flexible tubes 56 and 58.

Any gas suitable for handling through the plasma jet torch and inert to the metal of the ingot being conditioned may be employed to form the atmosphere in the container 10. Argon and/ or helium are preferred, and Helium will provide a desirable atmosphere for conditioning ingots of titanium, zirconium, and steel, as well as other refractory metals and their alloys.

It will be seen that operation of the plasma jet torch provides a flow of inert gas through a restricted space enclosed by container 10, this gas flowing over the surface of ingot 30 and out bleed line 14. Maintenance of a flow of inert gas over the surface of ingot 30 being conditioned prevents deleterious oxidation of the surface thereof during the time the metal is molten in the form of a puddle, and for at least long enough thereafter for the conditioned surface to be cool enough to expose without atmospheric contamination, that is formation of deleterious and undesirable combinations with atmospheric gases, including oxides and nitrides.

The plasma jet torch 46 is operated with a relatively low gas flow to form a plasma efliuent which is extremely hot yet does not penetrate too far into the metal of the ingot being conditioned. The gas flow will be appreciably lower than that required, for example, when such a torch is used for cutting and will in general be arranged to produce a flame similar in appearance to an oxyacetylene welding flame.

A plasma jet torch has normally two electrodes, one inside the body of the torch (often called the back electrode) and the other forming a nozzle to shape and constrict the plasma effluent. It is often desirable to provide an additional are between the nozzle electrode and the work, or the arc may be maintained between the back electrode and the work. We have found that good results during ingot conditioning can be obtained when the principal arc is between the nozzle electrode and the work (through contact shoe 32 and roller 18) with an are between the back and nozzle electrodes acting as an auxiliary. Good results may also be obtained with the back electrode in circuit with the work to provide the principal are between the back electrode and the work.

'I'he'speed of rotation of the ingot 30 on rollers 18 as well as the longitudinal travel of torch 46 along screw shaft 36 may be adjusted so that the puddle of molten metal is maintained on the ingot surface and caused to progressively and slowly travel around the circumference of the ingot in a helical path. It is necessary that adjacent convolutions of the puddle path overlap so that the ingot surface is effectively progressively melted and surface defects eliminated.

The angle 'at which the jet of arc plasma is directed toward the ingot surface is critical for best results. It is prefer-red that the angle of principal direction of the plasma jet should be between 2 and 8 degrees from a perpendicular to the general plane of ingot surface at the puddle area. Advantageously, this angle should be about 4 degrees from such a perpendicular as illustrated at A in FIG. 3. If the plasma jet is directed perpendicular to the ingot surface, the effluent from the plasma jet torch 'will tend to gouge out a crater in the surface rather than forming a puddle of most effective area and depth. If the angle of plasma jet direction to the surface perpendicu-lar is appreciably greater than 8 degrees, then the jet force will tend to blow the molten metal out of the puddle'. Good results have been obtained. by setting the arc plasma jet direction at about 4 degrees off perpendicular t'if the ingot surface since this produces effective melting to farm and maintain a puddle of molten metal without crateriitgf or gouging and without blowing the molten metal out of the puddle area. In the case of a curved surface, as the circumference of a cylindrical ingot, the reference perpendicular will by necessity be a general or average over the puddle area, and may conveniently and usefully be taken as a radius of the circle forming the ingot circumference and passing centrally through the puddle as illustrated in FIG. 3.

When conditioning a cylindrical ingot, the position of the puddle with respect to the ingot circumference is also critical for best results. With the ingot rotating horizontally around its longitudinal axis, the puddle is preferably maintained on the rising side of the ingot and advanta eously in the top octant of the rising side, this oetant being indicated in FIG. 3 of the drawings at B. The puddle should not be maintained on the falling Side of the ingot; indeed it may be extremely diflicult if not im possible to maintain a fixed puddle position on the falling side since gravity and the ingot rotation will cause the molten metal to run forward and fall out of the shallow puddle depression. Nor should the puddle position be too low on the rising side since the more nearly vertical surface angle will cause the molten puddle metal to run back in spite of the upward and forward movement of the ingot surface. When located in the top octant of the rising side, the puddle can be properly maintained without the molten metal running back and, by the time the melted metal has passed over the top of the ingot surface travel path, it will have frozen so that danger from running off and dripping when it reaches the falling side is eliminated. It will be appreciated that in a small ingot, of small diameter, the curvature of the ingot surface is sharper and the puddle location more critical. In the case of large diameter ingots, the surface curvature is correspondingly flatter and more leeway is available within the top octant of the rising side to locate and maintain the puddle for most effective operation.

The size of the puddle of molten metal on the ingot surface may vary from 1 to several inches in diameter. Good coverage and eflicient progressive fusion has been obtained using a puddle diameter of 2 to 3 inches, although larger diameter puddles may be found to be advantageous when conditioning large size ingots, or for speedier surface coverage. The depth of the puddle at its center should be sufficient to produce fusion of sufficient surface metal so that metal bounding or associated with defects is molten during puddle formation. Defects in most metallic ingots can be healed or eliminated by a quarter inch deep puddle, and more severe surface irregularities can most often be eliminated by a half inch puddle depth. It will be appreciated that a greater puddle depth will require greater power input and probably a larger torch.

Fusion conditioning of titanium metal ingots employing an arc plasma jet as described herein is effective to produce a well conditioned surface which will result in substantial economies in ingot processing. Table 1 below shows the comparative losses in converting ingots to sheet bar, using ingots of titanium metal as-cast, machine conditioned, and conditioned according to this invention- It will be noted that the as-cast ingots required a substantial amount of metal removal after forging to produce: sound bar, the machined ingotslost a substantial amount. due to metal removal on the lathe, while the arc plasma conditioned ingots lost no weight during the conditioning; step and little metal removal'after forging was required. It will be seen that the average yield from are plasma. conditioned ingots was 84.3 percent compared to 77' percent for as-cast and 74.6 percent for machine condi ti-oned ingots.

a ainst) TABLE 1 7 z l Grinding nd Conditioning Forging metal removed Crop loss, Xreld, 1 ss, ss, after percent percent percent percent; forging, percent p,

'853 2'4 *as' 82.5 mm 4.1 31.1 6.8 '86.

. .31 3.6 16.3 5.3 74.8 mm at 13.0; 4.5. 79.1 9.3 an 5.7 5: 6 76.4 11. 31 4.1 4. 1 7:7 72. 8

Adjusted for excessive pipe loss.

The following example illustrates an embodiment "of the practice of this invention:

Example 1 Apparatus of the type hereinbefore described and illustrated in the drawings is employed.

The door 16 of container; '10 is opened and an ingot 12 inches in diameter and 28 inches long of titanium alloy containing 6 percent aluminum and 4 percent vanadium is placed inside the container on supporting rollers 18 Door 16 is then closed and sealed and the valve in bleed line 14 is closed. The valve in pipe connection 12is opened and this connection placed in communication with a pipe line'from a vacuum pump to evacuate the interior of container 10. After evacuation, the vaccum pump is disconnected, the interior of container is back-filled with helium, and the valve on pipe connection 12 is closed.

The plasma jet torch'46 is mounted so that the arc plasma jet will rena e of about 4 degrees to a "perpendicular to the ingot surface at the contact area jto form a puddle close to t'he' top on the rising side of the ingot, and within the top rising oc tant of the ingot circumference. Helium is supplied "to torch '46 through supply line '54 and Water lines 56 and 58 are connected 'to supply cooling fluid to the torch. Power leads 50 and 52 are connected to asui'tablesou'rc'e of direct electric current to provide current at about 400 amperes and 32 volts between the nozzle electrode and the back electrode of torch 46. In addition, the nozzle electrode is connected through power lead 34 to another supply of direct electric current to provide 570 amperes at 37 volts between the nozzle electrode of torch 46 and ingot 30. The valve in bleed line 14 is then opened to vent excess helium entering container 10 through torch 46 and to provide a fiow of helium over the surface of ingot 30.

Rollers 18 are then actuated through the gears and by motor 28 to rotate ingot 3t horizontally around its axis at a speed of about /3 rpm. After one revolution to cover the ingot end area, motor 40 is started to rotate screw 36 to move torch mount 42 longitudinally along the length of ingot 30 a distance of 1 /2 inches for each complete revolution of the ingot.

The jet of plasma from torch 46 melts the titanium alloy metal on the surface of ingot 30 to form a puddle about inch deep at the center and about 2% inches in diameter. As ingot 30 slowly rotates, the puddle travels around the circumference thereof, and the longitudinal movement of torch 46 provides an overlap of about inch between each convolution of the helical path followed by the puddle around the circumference of ingot 30.

After the puddle, melted by the efliuent 48 from torch 46, has traveled to the end of ingot 30 and the last revolution made to complete surface coverage, motor 40 is shut down, then motor 28 is shut down to stop rotation of rollers 18, and the conditioned ingot 30 is allowed to cool in the container atmosphere and then removed through door 16.

The surface of ingot 30 will be free from cracks, porosity, loose metal, and other defects. The slight ridging or corrugations formed by the *adjacent travel paths of the puddle of metal during conditioning will not affect subsequent processing.

When conditioning iarge ingots, a plurality of plasma jet torches may be arranged to produce a plurality of puddles of molten 'metal moving simultaneously across the surface of the ingot. 'Such torches maybe disposed to cause such puddles to travel simultaneously adjacent paths or spaced paths, with the spaces between covered by later traveled paths produced by torch or ingot motion. The same criteria apply for multiple torch operation as for the single torch operation described hereinbefore in more detail; namely, that adjacent puddle travel paths must "overlap and that the puddles preferably be formed andmaintained within the top octant of the rising side of a cylindrical ingot. In addition, the angle of the effluent from each of the multiple torches should preferably be from 2 to 8 degrees from the perpendicular to the ingot surface "as described. 7 H v I Y n Considerable heat is applied to the ingot surface during conditioning while the metal in the interior will remain relatively cold. Under these conditions, hoop stresses are set up in the surface layer of ingot metal when this cools. Such "surface stresses may in the "case of some 'm'etals or'specific alloys cause surfacecracks to form on cooling. Such cracking may 'be avoided by first preheating the. ingot to an elevated temperature, preferably between l000 and 1500 F., and conditioning while hot. Care should be taken that the ingot is soaked long enough at temperature to insure that the entire ingot, including the center, is raised to annealing temperature. This step introduces a condition wherein the center as well as the outside surface will cool after conditioning and reduces the effect of cooling induced contraction occurring only in the surface layer.

Alternatively or additionally; the ingot may be conditioned in an initially cold state and immediately after conditioning before cooling is given a stress relieving post-anneal. This is accomplished by heating the entire ingot to a suitable annealing temperature, depending on its metal or alloy composition, which will generally be from 1200 to 1500 F. After such heating, the ingot is slowly cooled.

The pre-heating or post-annealing step or both may be employed as an initial step or later step added to the general process described herein and as illustrated specifically in Example. 1.

Conditioning ingots employing an arc plasma for progressive fusion of the surface metal has several advantages over other methods. No metal is removed as in conditioning by machining so that substantially the entire weight of ingot metal cast is available for subsequent processing. While machine chips may be remelted and thus recycled, this does not by a wide margin make up for the loss since handling, collecting, cleaning, mixing, and remelting involve considerable expense even though a major part of the machine chips may be recovered. Such costs are completely avoided when are plasma conditioning is employed.

The nature of the heating source in the method of this invention is important to produce effective fusion Without encountering other difficulties. The are plasma torch provides an uniquely advantageous heating source providing efiective heating over a reasonable area. It can be adjusted to provide a diffused heating zone rather than the concentrated heating effect of a normal arc, and can readily be arranged to produce a practical size puddle which can be made to travel in a uniform pattern over the ingot surface. The jet of plasma issuing from the torch can be controlled, particularly as to direction so that, as described hereinbefore, the stream of plasma at high velocity can be adjusted and directed to produce an effective melting puddle without gouging or cratering. Even so, the heat of the arc plasma is sufficiently intense to provide adequate heat for fusion of even highly refractory metals.

Another unique aspect of use of a plasma jet for ingot conditioning is that the inert gas employed to produce the plasma jet by its passage through the torch and the are also serves, at least in part, as the protective, inert gas maintained flowing over the ingot surface to prevent atmospheric contamination. Depending on the precise apparatus design, additional inert gas may be required to provide adequate flow, or for cooling the ingot surface, but a principal source of such gas is from the plasma torch efiiuent.

We claim:

1. A method for conditioning the surface of an ingot of refractory metal which comprises:

(a) heating a limited area of the surface of said ingot without removal of metal of said ingot by impingement thereon of a jet of arc plasma to form a puddle of molten metal on the surface of said ingot;

(b) causing relative motion between a source of said are plasma and said ingot to advance the said puddle of molten metal by progressive melting and solidification of increments of the metal of said ingot along a path across a surface of said ingot; and

(c) continuing to cause relative motion between the source of said are plasma and said ingot to advance said puddle of molten metal along a path on said surface of said ingot which overlaps a path over which a puddle had been previously advanced;

meanwhile,

(d) maintaining a flow of inert gas through a restricted space. over the said surface of said ingot to prevent atmospheric contamination.

2. The method of claim 1 in which the said ingot is cylindrical and the said path of said puddle of molten metal is helical about the surface of said ingot.

3. The method of claim 1 in which the said jet of arc plasma is directed to impinge on the said ingot at an angle of between 2 and 8 degrees to a perpendicular to the surface thereof.

4. The method of claim 1 in which the jet of said are plasma is directed to impinge on the said ingot at an angle of about 4 degrees to a perpendicular to the surface thereof.

5. The method of claim 1 in which the said ingot is cylindrical and is rotated horizontally about its axis and the said jet of arc plasma is directed to form the said puddle of molten metal on the rising side of said ingot surface.

6. The method of claim 1 in which the said ingot is cylindrical and is rotated horizontally about its axis and the said jet of arc plasma is directed to form the said puddle of molten metal in the top octant of the rising side References Cited by the Examiner UNITED STATES PATENTS 2,059,236 11/1936 Holslag 2l969 2,778,926 1/1957 Schnekler -10 2,959,503 11/1960 Lindson l48-33 3,050,616 8/1962 Gage 2l9-69 3,086,856 4/1963 'Siebertz 7510 DAVID L. RECK, Primary Examiner. 

1. A METHOD FOR CONDITIONING THE SUFACE OF AN INGOT OF REFRACTORY METAL WHICH COMPRISES: (A) HEATING A LIMITED AREA OF THE SURFACE OF SAID INGOT WITHOUT REMOVAL OF METAL OF SAID INGOT BY IMPINGEMENT THEREON OF A JET OF ARC PLASMA TO FORM A PUDDLE OF MOLTEN METAL ON THE SURFACE OF SIAD INGOT; (B) CAUSING RELATIVE MOTION BETWEEN A SOURCE OF SAID ARC PLASMA AND SAID INGOT TO ADVANCE THE SAID PUDDLE OF MOLTEN METAL BY PROGRESSIVE MELTING AND SOLIDIFICATION OF INCREMENTS OF THE METAL OF SAID INGOT ALONG THE PATH ACROSS A SURFACE OF SAID INGOT; AND (C) CONTINUING TO CAUSE RELATIVE MOTION BETWEEN THE SOURCE OF SAID PLASMA AND SAID INGOT TO ADVANCE SAID PUDDLE OF MOLTEN METAL ALONG A PATH ON SAID SURFACE OF SAID INGOT WHICH OVERLAPS A PATH OVER WHICH A PUDDLE HAD BEEN PREVIOUSLY ADVANCED; MEANWHILE, (D) MAINTAINING A FLOW OF INERT GAS THROUGH A RESTRICTED SPACE OVER THE SAID SURFACE OF SAID INGOT TO PREVENT ATMOSPHERIC CONTAIMINATION. 